** This thread discusses the Content article: Machining Piston to Avoid Seizures **

The air cooled engine suffer worse seizure issues because they run so much hotter. If the fan on engine cannot keep up with the heat created, the piston will continue to get hotter yet and continue expanding within bore untill it runs out of clearance.
Same as the theory you say is incorrect, just the cause is different, end result remains the same.

JMO,
Scott

Scott,

As always your opinion is respected and duly noted. My opinion is that most people believe that the air-cooled engines do not have these problems and that is as far from the truth as possible. The fan on these engines was designed for these particular engines and will produce a adequate amount of air even at very high rpm stressful situations as I have tested our engine with a machined piston to 20,000+ rpm with no problems in the very hot Louisiana weather. Without the modification the results would have been disastrous and expensive. End result is the piston needs to be turned because of heat expansion on the crown of the piston. Water cooled, air cooled it does not matter the most cost effective simple fix is to turn the top diameter and have one less problem to worry about.

Carlo,
We are on the same page .... As you have noted the area that expands most is indeed the piston dome. The mod we do under cutting the diameter around ring is our ONLY solution to an engineering issue NOT addressed by the manufacturers.
In High Performance engines built for RACING duty, pistons are machined either tapered dome to skirt, And / Or cam ground that makes them No Longer Round. Cam grinding is done for the SAME REASON as our diameter turning. The material is thicker around a pistons pin boss area and therefor expands more than the thinner thrust surfaces 90* to either side of them.

BTW, Great seeing a legitimate engine tuner talking about things that get over looked or ignored due too ignorance of such things.

Regards, Scott

Does that mean it would be legal to turn the piston in the Classic Thunderboat class?

Machining ANYTHING on the engine would make it not stock and would violate the thunderboat rules as written. My thunderboat engine has 50 dyno runs and 4 heats of racing with no sign of piston problems.

Lohring Miller

lohring wrote:
Lohring,
Consider yourself one of the lucky ones as I have repaired more engines then I can count from this problem. Some of them never have a problem and others happen right away. It is just luck of the draw. In my opinion those rules should be adjusted to allow that modification only.

Maybe the dyno runs broke in the piston before the racing heats. The runs are very short and nothing has time to get hot. I advocated for G1 engines for the class in the Propwash. This would eliminate a lot of the fights and stupid disqualifications I see coming. Years of running 'stock' outboards gives me a dislike of stock engine rules. In 2002 the NAMBA Nationals B Stock Tunnel winner was disqualified for a part substitution. Don't change your Zenoah bolts or water fittings.

Lohring Miller